I would think the extension card would work fine. You may have an address problem with it though. Are you using Mach3?
You can also use a USB interface like the smoothstepper.
Warp9 Tech. Design - Home of the SmoothStepper - Home
I have several of these and they work great. Your machine will run smoother and faster than you have ever seen.
I'm sure others with more knowledge of this will chime in with more definite answers, but those are the 2 things I would look at.
OB Cue Shafts and OB Cues